I.Mear One's mural was antisemitic. If Trump posted an image of that mural on his Twitter feed, it would immediately be recognised for what it is. It is the idea that it is in some way associated with the Left, promoted by the artist himself and by some of those who offer him platforms, that has caused confusion.

This means that what the Left says about antisemitism matters, because it can either marginalise it or normalise it among people who listen to what the Left is saying. Lutfur Rahman was right to have it painted over. Corbyn was wrong to have defended it in a Facebook comment at the time, and he's rightly embarrassed about it and trying to fix it. It's the least that any leader of the Left should do. I am not suggesting his politics should be judged on the basis of an offhand Facebook comment, but he's right, and not pusillanimous as some think, to address it at the scale that he is.

Nothing would be worse, at this moment, than for the Left to adopt the crankish, defensive and pointlessly antagonistic rhetoric of some of those organising 'against the witch hunt'. I have no doubt that many of Corbyn's critics are recklessly opportunistic, and instrumentalising the issue for ends that aren't to do with reducing antisemitism. I also think it ludicrous that media reports have consistently inflated unrepresentative examples of antisemitic behaviour into evidence of systemic, pervasive antisemitism in Labour -- on which issue, the Jewish Labour Movement has made some constructive statements. (Also listen to Jamie Stern-Weiner.)

But that isn't a reason to dodge this, or the wider issue. At a time when various forms of fascism and populist reaction are on the rise, antisemitism risks acquiring a degree of political clout it hasn't had for decades. The Left has a unique responsibility to address and combat this, both in itself and as part of a general increase in racism. It would be unable to do so if it was tying itself up in knots defending the indefensible.

II.Antisemitism is not particularly worse on the Left than elsewhere. My source for this is none other than the Community Security Trust, and its study of the political distribution of antisemitism. I take this study, with all due caveats about the loaded and question-begging nature of some of its approach, as a case of evidence-against-interest. Those liberals who think of antisemitism as a problem of the 'extremes' are wrong. The evidence suggests that it's a problem primarily of the Right, but no one is innocent (and therefore, the tacit self-congratulation of liberal 'horseshoe' theories of antisemitism is unwarranted).

But this also means the Left is not exempted from antisemitism. Indeed, the CST asserts -- and this does not seem prima facie implausible -- that if the Left is not more antisemitic than the political 'centre-ground', it is also not less antisemitic. We should think about what that means. Would we be dismayed to learn that people who identified with the Left were no less racist than the mean when it came to anti-black racism, or Islamophobia? Would it raise questions about how the articulate, organised Left had handled these issues?

Most antisemites on the Left are marginal, not particularly powerful or prominent. There have certainly been pungent conspiracy-minded countercultures on the fringes of the Left, especially since 9/11 -- the Truthers being the legitimate ancestors of today's chemtrail left. But it's easy to ignore these sorts, since they wield no power. However, in the course of my own political life, I've seen a number of examples of tactical adaptations to antisemitism.

This would include promoting Gilad Atzmon on the phoney prospectus of influencing his politics, or on the basis of obtusely 'not noticing' his antisemitism. Or being quietly uncritical of Tam Dalyell's antisemitic intervention about a "cabal of Jewish advisers", because acknowledging it would only reinforce the purveyors of a "new antisemitism" thesis. (To the contrary, ignoring it when it's right in front of you is what reinforces that thesis.) Or, most egregiously I think, Counterpunch publishing an actual Holocaust-denier and blood libelist, Israel Shamir, presumably on the basis of its disastrous conception of a left-right alliance against war.

In all of those cases, it is difficult to imagine those groups not only tolerating but, in some cases, promoting overt and pungent racism of any other type in that way. I'll come back to why that is.

III.As follows from the above, antisemitism may not be more prevalent on the Left, but there are specific types, articulations, which I would assert from experience are more likely to appear on, or in the peripheries of, the Left. For example, it may come in the form of a conspiracy theory about "the elite", or in the form of a certain kind of anti-Zionism. August Bebel dubbed antisemitism of this kind the "socialism of fools", because it was offered as an ersatz, racist theory of class and political domination.

Beyond these types of overt antisemitism, there is a kind of tone deafness that is probably particular to the Left, and comes with a sense of brittle self-righteousness. Think of a long-standing anti-Zionist activist choosing a Jewish Voice for Labour meeting to defend the right to 'free speech' about the Holocaust. Think of a Jewish socialist ranting about "Zios". Think of an experienced socialist, rebutting claims of antisemitism by saying "I've never seen any". Think of a former mayor of London firing off about the Havaara agreement in the worst possible way, and pointedly not taking any criticism. Think of people gatecrashing a Jewish Labour Movement meeting to make a futile intervention against the 'Zionist lobby'.

It's not that any of these people are antisemites, although some of them have become bunkered and cranky through years of political isolation. Rather, some people have cultivated a kind of gratuitous and performative political 'toughness', and defend this fragile 'toughness' as if it was the same thing as rigour and hard-headedness. And in a context in which we have not seen organised antisemitism of any significant scale for a long time, and in which those with democratic and internationalist objections to Zionism have often been stigmatised as antisemitic, it has been easy for some people to become dismissive of the whole issue. But that complacency was wrong then, and it is a liability now.

Now is a good time for a collective upping of our game, and for a certain realignment of sensibilities and tolerances. Meaning, we have to marginalise the cranks, and the racists, and have very 'hard arguments' with those who humour them. Not in a defensive response to hyperbolic attacks, not to please the media, not to court favour with mainstream opinion, but because it would be absurd not to.

As far as I can see, these are the terrestrial realities. Those who wish to refute them will find it cold in outer space: they should come back down to earth.